Amplitude modulation (AM) is a common modulation technique, and for a digital communication system, amplitude shift keying (ASK) is a common AM technique. For example, an RFID system employs the ASK technique to modulate and demodulate transmission data. When AM is performed, a to-be-transmitted baseband data signal is modulated by a carrier signal to generate an AM signal, and amplitude of the AM signal represents the transmission data. For example, when the transmission data are digital data, low amplitude represents a digital “0”, and high amplitude represents a digital “1”. In the AM technique, carrier signals are often sources of noises, and thus a signal-to-carrier ratio needs to be increased to gain better demodulation effect when the AM signal is demodulated.
In the prior art, when the AM signal is demodulated, an all-poles low-pass filter is applied to pass baseband data signals and simultaneously attenuate high-frequency (HF) carrier signals thereby increasing the signal-to-carrier ratio. When a frequency of the data signal is far lower than a carrier frequency, only a low-stage low-pass filter is needed to gain an acceptable effect. However, when the frequency of the data signal is increased, i.e., a data rate of the communication system is increased, a difference between the frequency of the data signal and the carrier frequency is reduced. In particular, in a communication system having a low carrier frequency, e.g., a low-frequency (LF) or an HF RFID system, the difference between the frequency of the data signal and the carrier frequency will be further reduced when the data rate is increased. Under such a situation, the low-pass filter having a higher stage is needed to pass the data signal and simultaneously attenuate carrier signals. However, complexity and cost of circuit design are accordingly increased.
For example, FIG. 1 shows frequency response of a single-pole low-pass filter, where a horizontal axis represents frequency and a vertical axis represents a gain value. Supposing that a carrier frequency fc is 4 times of a baseband frequency fs, which is equal to a cutoff frequency corresponding to a gain value of −3 dB of the single-pole low-pass filter. Since a stop band of the single-pole low-pass filter has a slope of −20 dB/decade, the carrier frequency corresponds to a gain value of −12 dB. Assume an n-stage all-poles low-pass filter is needed to achieve a signal-to-carrier ratio R that is higher than 30 dB. Accordingly, the signal-to-carrier ratio R can be represented as Formula 1:R=n*(−3 dB)−n*(−12 dB)>30 dB  (1)Accordingly, it is obtained that n≧4, i.e., a 4-stage all-poles low-pass filter is needed to achieve the signal-to-carrier ratio R that is higher than 30 dB. However, complexity and cost of circuit design are accordingly substantially increased.